Overspeed indicator for a rotatable shaft

ABSTRACT

An overspeed indicator for a rotatable shaft comprises, in the example disclosed, a circular disc of inexpensive, frangible material such as chip board, paper board, or card board. The disc has a central, polygonal opening enabling it to be screwed onto a threaded end of a shaft as a temporary substitute for a rotatable member such as a grinding wheel normally supported on it. The disc also has an off-center opening within which there is a cantilever overspeed tab integral with the body of the disc extending radially inwardly from an outer peripheral edge portion of the disc and parallel to the fibrous grain of the frangible material. The tab is designed to fly off, harmlessly, when rotated at a sufficiently high speed above the normal rating of the grinding wheel to indicate it would be hazardous to use the wheel at that speed. Non-fibrous material having little or no grain may be substituted for chip board, one example being given as &#39;&#39;&#39;&#39;STYROFOAM&#39;&#39;&#39;&#39; foamed polystyrene sheet material.

United States Patent [191 Perham et a1.

[ OVERSPEED INDICATOR FOR A ROTATABLE SHAFT [75] Inventors: James A. Perham; Bernard N. Hill,

both of Sioux City, Iowa [73] Assignee: Sioux Tools Inc., Sioux City, Iowa [22] Filed: May 24, 1973 [21] Appl. No.: 363,371

[52] US. Cl 116/74, 73/488, 116/114 [51] Int. Cl. G08b 21/00 [58] Field of Search 116/73, 74, 114, 115;

[56] References Cited UNITED STATES PATENTS 547,280 10/1895 Nicholson ..l16/74 Primary Examiner-Louis .l. Capozi Apr. 30, 1974 ABSTRACT An overspeed indicator for a rotatable shaft comprises, in the example disclosed, a circular disc of inexpensive, frangible material such as chip board, paper board, or card board. The disc has a central, polygonal opening enabling it to be screwed onto a threaded end of a shaft as a temporary substitute for a rotatable member such as a grinding wheel normally supported on it. The disc also has an off-center opening within which there is a cantilever overspeed tab integral with the body of the disc extending radially inwardly from an outer peripheral edge portion of the disc and parallel to the fibrous grain of the frangible material. The tab is designed to fly off, harmlessly, when rotated at a sufficiently high speed above the normal rating of the grinding wheel to indicate it would be hazardous to use the wheel at that speed. Non-fibrous material having little or no grain may be substituted for chip board, one example being given as STYROFOAM foamed polystyrene sheet material.

10 Claims, 6 Drawing Figures OVERSPEED INDICATOR FOR A ROTATABLE SHAFT BACKGROUND OF THE INVENTION The use of rotatably driven grinding or abrasive wheels, buffing wheels, burring devices, power take-off wheels, and similar members mounted on rotatable shafts is widespread and the advantages as well as the problems of using them are well known in our industrial society.

One of the more serious problems encountered is to protect the operator if the rotating member flies apart in use. Grinding wheels rotate at very high speeds. Wheels of six to eight inch diameter operate at 3,600 to 6,000 revolutions per minute. Smaller wheels in some cases rotate up to 50,000 rpm. These high speeds, and the relatively brittle abrasive ingredients in most grinding wheels make a hazardous combination even where rubberlike or fiberglas matrixes are used.

Generally, for every rotatable member such as a grinding or buffing wheel, there is an optimum speed range which is best for safe and effective operation, and there is some higher speed which is dangerous to the operator and others in the vicinity because the rotating member may fly apart by centrifugal force.

Recognizing this danger, it is established practice many shops to check grinder speeds daily with a tachometer to be sure they are well within safe operating ranges. As additional precautions, guard surround the grinding wheels, and operators and other personnel in the area wear safety goggles. Grinding and buffing wheels are made in many types, some for general purpose work, and some for special grinding and buffing at different speeds. For example, a common, low cost, general purpose grinding wheel is quite rigid and designed for safe use up to 3,600 rpm. A special purpose burnishing or polishing wheel may have a flexible rubber-like or cloth binder and be designed for use at much higher speeds, say 6,000 rpm wheel on a spindle driven at 6,000 rpm. There is the ever present possibility, where a shop has many grinding spindles, both hand-held and machine-mounted, and different ones driven at different speeds, that someone may inadvertently try to use a slow speed wheel on a high speed spindle and cause it to explode in his face.

SUMMARY OF THE INVENTION This invention relates generally to rotatably driven members, particularly wheels and the like, and to improved means enabling an operator to test a drive shaft for such a member and verify that it rotates within a safe speed.

It is a general object of the invention to provide an overspeed indicator for a rotatable shaft which is easy to use by the least skilled operators, and is so inexpensive that it may be kept readily available as a safety device to all who use high speed rotating assemblies such as grinding wheels.

A specific object of the present invention is to provide a disc of inexpensive frangible material such as chip board, card board, pulp board or foamed plastic which can be fitted on a grinder shaft in place of a grinding wheel, and spin-tested to determine if the shaft speed is safe.

A further object is to provide such a disc of chip board or the like with a continuous circular periphery,

a central polygonal opening designed to be screwed temporarily onto the threaded end of a grinder spindle to replace a grinding wheel, and a cantilever, overspeed tab in an off-center opening integral with the disc and extending radially inwardly from an outer peripheral portion of the disc, the tab being dislocatable by centrifugal force in response to rotation of the shaft at a predetermined speed above a safe range for the grinding wheel.

Another object is to provide such a chip board safety disc which can be constructed in different configurations and of different materials for use with grinding wheels operating at different speeds.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages will be apparent from the following description taken in connection with the drawings in which:

FIG. 1 is'a side view, with portions in cross section, illustrating a rotatable drive shaft for a grinding wheel with which the present invention may be used;

FIG. 2 is a plan view of a chip board disc illustrating one preferred form of the invention;

FIG. 3 is a perspective view of a spacer which is shown assembled in FIG. 4;

FIG. 4 is a view similar to FIG. 1, with the overspeed disc shown in FIG. 2 and the spacer shown in FIG, 3 substituted for the grinding wheel;

FIG. 5 is a fragmentary, enlarged view of FIG. 4 as seen in the direction of the arrows 55; and

FIG. 6 is a fragmentary view similar to FIG. 2 showing the manner of dislodgement of the overspeed indicator tab by centrifugal force at a speed at which the tab is designed to be dislodged.

Like parts are designated by like reference characters in the various figures.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a grinder assembly with which the overspeed indicator disc of the present invention may be used. An abrasive wheel member 20 is detachably mounted on one end of a rotatable drive shaft or spindle 22 which is rotatably journaled by bearing means (not shown) within a housing 24. A drive flange 26 is supported on the housing 24 and encircles the outer end of the spindle 22. A metal or plastic hub member 28 has a cylindrical body portion 30 extending through the wheel member 20 and has a beaded outer end portion 32 projecting into the central depressed cavity 34 of the wheel and grips an annular metal or plastic liner 36 which extends through the central opening of the wheel. The wheel is assembled with the cylindrical body portion 30 of the hub member encircling the threaded end portion 38 of the spindle 22. The inner end of the hub is seated against the surface 40 of the drive flange 26 and is held in place by a nut 42 screwconnected to the threads 38. A lock washer 44 is interposed between the nut 42 and bead 32.

The overspeed indicator member is generally designated 46. It comprises a flat disc or body 48 having a continuous circular periphery 50 formed by punching or fabricated in any other suitable way from a low density frangible sheet such as that variously referred to in the paper industry as chip board, card board, or paper board; or a foamed plastic sheet such as STYROFOAM brand foamed polystyrene available from The Dow Chemical Company. The specific embodiment disclosed here is standard chip board available from any commercial paper product supplier and is made from reprocessed corrugated box clippings and book scrap, having a moisture content of 5-5 /2 percent. Standard chip board has a grain extending parallel to the fibers as indicated by the broken lines 66 in FIGS. 2 and 6. Foamed plastics materials are cellular and have little or no grain so are substantially the same strength in all directions.

The disc 48 is formed with a central polygonal opening 52 with an internal dimension enabling it to be screwed onto the spindle threads 38 as shown in FIG. 4. In one specific example, where the threads 38 have an outside diameter of five-eighths inch, the opening 52 is hexagonal, and one-half inch across the flats.

In addition to the central polygonal opening described, the disc body 48 is formed with an off-center opening 54 adjacent the circular periphery. Within the off-center opening is a cantilever tab 56 which is integral with the body and extends radially inwardly from the outer peripheral portion 58, and preferably in the direction of the fibrous grain 66, where chip board ma- .terial is used.

The off-center opening 54 in the example shown is U-shaped, having a pair of parallel slots 60, 60 joined across the bottom by a curved connecting slot 62. The parallel slots 60 extend generally radially outwardly from the center of the disc body and define the radially inwardly extending tab 56. The base of the tab is the outer peripheral portion 58 of the disc body and the radial dimension between the tips or ends 61 of the parallel slots 60 and their periphery 50 determines the radial thickness b of the tab base 58. This, in turn, is one factor determining the break-out strength of the tab and the rotational speed at which it will be dislodged by centrifugal force.

The rotational speed causing the tab to break off can be varied and predetermined by proper selection of certain critical dimensions in the tab and base portion, and in selecting material of the desired strength. Where chip board is the material used, the most uniform break-up speed is obtained by making the tab parallel to the fibrous grain 66, as stated above. Referring to FIG. 2, critical parameters and dimensions determining the break-up speed are the tab length l and width w, the base dimension b measured radially of the disc, the thickness of the body stock as measured in the direction of the rotational axis, and the direction of grain 66 if fibrous material such as chip board is used.

Specific examples of overspeed indicating discs made of chip board in accordance with the present invention, and test evaluations of them, will now be described. Discs rated for use with 4,500, 6,000, and 7,000 rpm grinder spindles may be made as follows. 4,500 RPM RATING Use chip board sheet stock about 1/32 inch (0.03125 inch) thick. Cut or punch the disc as shown in FIG. 2, with an outside diameter of 7 inches and the U-shaped off-center opening 54 defining the following dimensions for the overspeed tab '56:

Width w 9/ 16 inch Lengthl= l-l/l6 inches Base b 9/32 inch 6,000 RPM RATING This may be made from the same chip board stock as the 4,500 rpm disc described above with the same dimension for the off-center opening 54 and tab 56, the only difference being that the base b is 15/32 inch instead of 9/32 inch.

7.000 RPM RATING This may be made from chip board material similar 'to that described for the 4,500 and 6,000 rpm discs, but

ported as follows:

Average speed RPM range where RPM range where where tab is of tabs 90% of tabs Rating of located for will break away will break away disc manubreakout Average Extreme factured as (average for relative relative described test samples) humidity humidi Above (0% TO l00%) 4500 5000 RPM 4800-5200 RPM 4600-5400 'RPM 6000 6650 RPM 6400-6900 RPM 6120-7180 RPM 7000 7700 RPM 7400-8000 RPM 7100-8300 RPM It has been determined in these tests that, although relative humidity does effect the break-out speed, that is, the speed at which thetab flies off, it is always in the direction of a lower rpm condition. This means that although there may be times when a tab breaks away when the grinder shaft is rotating at a proper speed, from a safety point of view, it is no hazard. It means at worst that the operator may unnecessarily be checking the speed of the grinder with a tachometer at times when the grinder is actually running at proper speed.

The overspeed indicator disc is not intended as a substitute for a tachometer, which should be used regularly to check all grinding, buffing, and polishing shafts and the like. Many shops make such tachometer checks once a day as part of their regular safety routine. An advantage of the overspeed indicator disc is that it is so cheap (three to four cents apiece in manufacturing quantities) that each operator can have a set in his own tool chest for instant checking of a grinder shaft where there is any question about the proper spindle speed, without wasting the time required to obtain the shop tachometer.

Use and operation of the overspeed indicator disc is believed obvious in view of the foregoing description. Briefly, assume an operator wants to use a grinder such as that shown in FIG. 1, but first wants to check the spindle speed for safety with a 4,500 rpm grinding wheel. To do this, he backs off the nut 42 and removes the washer 44 so he can release the wheel assembly 20 and remove it. He thenselects a 4,500 rpm rating overspeed indicator disc (as specified above) and replaces the wheel assembly 20 with a spacer tube 64 and then fits the center polygonal opening 52 of the 4,500 rpm disc over the threaded end portion 38 of the spindle 22.

By pressing and rotating simulataneously, he screws the chip board disc onto the threads until it stops against the end of the spacer tube 64 as shown in FIGS. 4 and 5. The disc will be held there securely, though temporarily, for the duration of the ensuing test rotation. The motor (not shown) driving the spindle 22 will then be turned on and the spindle will be brought up to full speed, after which the motor will be turned off and the spindle allowed to-coast to a stop. If the tab 56 is then intact, the spindle is safe to use with a 4,500 rpm grinding wheel. If the tab is gone, the spindle speed should be checked with a tachometer before using the grinder.

The spacer tube 64, like the disc 46 itself, may be made of relatively inexpensive, light-weight material such as chip board or foamed plastic. It will preferably be light enough that it can cause no hazard if it breaks or flies off.

For best results, the tab 56 should be pointed radially inwardly, as shown. Where chip board is used, the tab should be oriented the same direction relative to the fibers, preferably parallel to the grain 66. Centrifugal force will then break it off cleanly and completely, for example in the manner shown in FIG. 6 where there is a somewhat schematic illustration of the tab flying off under centrifugal force. By pointing the tab radially inwardly, this provides quite consistent break up speeds as shown in the chart above and, because the dislodged tab rotates in space as it dislodges, it flies only a short distance at low velocity and is harmless when it bounces off the surrounding guard. Where the disc is made of non-fibrous material such as STYROFOAM foamed polystyrene having no grain, there is of course, no concern with aligning the tab with a fiber axis 66.

While one form in which the present invention may be embodied has been shown and described, it will be understood that various modifications may be made within the spirit and scope of the invention which should be limited only by the appended claims.

I claim:

1. An overspeed indicator for a rotatable shaft comprising:

a disc-like body of frangible material having means to attach said body to the shaft for rotation therewith about a common axis;

said body having a weakened portion defining a tab which is dislocatable by centrifugal force in response to rotation above a predetermined speed.

2. An overspeed indicator for a rotatable shaft according to claim 1 in which said weakened portion includes an off-center opening near the periphery of the body, said frangible material is chip board, and said tab is a cantilever member integral with said body in said opening and extends radially inwardly toward the center of the body.

3. An overspeed indicator for a rotatable shaft according to claim 2 in which said tab is parallel to the fibrous grain of the chip board material.

4. An overspeed indicator for a rotatable shaft according to claim 1 in which said tab is defined by a generally U-shaped opening in said body having open arm portions pointing radially outwardly from the center of rotation of the body to thereby connect the tab and body across a base portion between the tips of said open arm portions and the outer periphery of the body.

5. An overspeed indicator for a rotatable shaft according to claim 1 in which said means comprises a polygonal center opening adapted to be screwed onto a threaded shaft.

6. An overspeed indicator for a rotatable shaft comprising:

a flat, disc-like body of frangible material having means for assembling on said shaft for rotation about a common axis;

said body having an off-center opening;

a tab in said off-center opening having a base portion integral with the periphery of the body at the outer portion of said opening and extending radially inwardly toward the center of said body;

said tab and base portion having selected dimensions enabling said tab to break off by centrifugal force in response to rotation in a predetermined speed range. 7. An overspeed indicator for a rotatable shaft according to claim 6 in which said means comprises has a center opening for said shaft.

8. An overspeed indicator for a rotatable shaft according to claim 6 in which said frangible material is Chip board.

9. An overspeed indicator for a rotatable shaft comprising a disc made of chip board sheet material having a continuous circular periphery;

said disc being formed with a central polygonal opening adapted to be screwed onto a threaded end of a shaft for rotation therewith about a common axis;

said disc being formed with an off-center opening adjacent the periphery and having within said opening a cantilever tab integral with the disc and extending radially inward from the outer peripheral portion of said disc parallel to the fibrous grain of the chip board sheet material; and

said tab being dislocatable by centrifugal force in a predetermined speed range depending on the dimensions of the tab and of said peripheral portion and the strength of the said chip board sheet material.

10. An overspeed indicator for a rotatable shaft comprising: a body having means to attach said body to the shaft for rotation therewith about a common axis; said body having a portion thereof which is dislocatable by centrifugal force in response to rotation above a certain 

1. An overspeed indicator for a rotatable shaft comprising: a disc-like body of frangible material having means to attach said body to the shaft for rotation therewith about a common axis; said body having a weakened portion defining a tab which is dislocatable by centrifugal force in response to rotation above a predetermined speed.
 2. An overspeed indicator for a rotatable shaft according to claim 1 in which said weakened portion includes an off-center opening near the periphery of the body, said frangible material is chip board, and said tab is a cantilever member integral with said body in said opening and extends radially inwardly toward the center of the body.
 3. An overspeed indicator for a rotatable shaft according to claim 2 in which said tab is parallel to the fibrous grain of the chip board material.
 4. An overspeed indicator for a rotatable shaft according to claim 1 in which said tab is defined by a generally U-shaped opening in said body having open arm portions pointing radially outwardly from the center of rotation of the body to thereby connect the tab and body across a base portion between the tips of said open arm portions and the outer periphery of the body.
 5. An overspeed indicator for a rotatable shaft according to claim 1 in which said means comprises a polygonal center opening adapted to be screwed onto a threaded shaft.
 6. An overspeed indicator for a rotatable shaft comprising: a flat, disc-like body of frangible material having means for assembling on said shaft for rotation about a common axis; said body having an off-center opening; a tab in said off-center opening having a base portion integral with the periphery of the body at the outer portion of said opening and extending radially inwardly toward the center of said body; said tab and base portion having selected dimensions enabling said tab to break off by centrifugal force in response to rotation in a predetermined speed range.
 7. An overspeed indicator for a rotatable shaft according to claim 6 in which said means comprises has a center opening for said shaft.
 8. An overspeed indicator for a rotatable shaft according to claim 6 in which said frangible material is chip board.
 9. An overspeed indicator for a rotatable shaft comprising a disc made of chip board sheet material having a continuous circular periphery; said disc being formed with a central polygonal opening adapted to be screwed onto a threaded end of a shaft for rotation therewith about a common axis; said disc being formed with an off-center opening adjacent the periphery and having within said opening a cantilever tab integral with the disc and extending radially inward from the outer peripheral portion of said disc parallel to the fibrous grain of the chip board sheet material; and said tab being dislocatable by centrifugal force in a predetermined speed range depending on the dimensions of the tab and of said peripheral portion and the strength of the said chip board sheet material.
 10. An overspeed indicator for a rotatable shaft comprising: a body having means to attach said body to the shaft for rotation therewith about a common axis; said body having a portion thereof which is dislocatable by centrifugal force in response to rotation above a certaiN speed. 